Subsets

class Solution {
public:
vector<vector<int> > subsets(vector<int> &S) {
vector<vector<int> > ret;
size_t len=S.size();
if(len < 1) return ret;
sort(S.begin(),S.end());
const size_t sum=pow(2.0,len);
for(size_t i=0;i<sum;i++)
{
vector<int> unit;
unsigned long long flag=1;
for(int times=0;times<len;times++)
{
if(flag&i)
{
unit.push_back(S[times]);
}
flag=flag<<1;
}
ret.push_back(unit);
}
return ret;
}
};